Digital systems are commonly embedded on printed circuit boards. Different integrated circuits positioned on a printed circuit board may operate at different voltages. For example, with improvements in process technology, integrated circuits use lower power supply voltages, such as 3.3 Volts or 2.5 Volts, or even lower. Integrated circuits made with these processes should remain compatible with previous generations of integrated circuits. For example, a new generation 3.3 Volt integrated circuit may need to be used on a printed circuit board with an old generation 5 Volt integrated circuit. Systems of this type are commonly referred to as mixed-voltage systems.
The 3.3 Volt integrated circuit will need to have the proper supply and input voltages for operation. In addition, the 3.3 Volt integrated circuit should supply or generate the proper output voltages for interfacing with the other integrated circuits. Proper interfacing of the integrated circuits is essential for proper functional operation. Further, proper interfacing will prevent undesirable conditions, such as overstressing the devices, avoiding possible high current or latch-up conditions, and other similar concerns, thereby improving device longevity.
Some circuit architectures rely upon separate noisy and quiet supply voltage schemes. For example, an I/O driver may be coupled to a noisy supply while the on-chip conversion circuitry is coupled to the quiet supply. This will provide some isolation from noise at the I/O driver from being coupled to other on-chip circuitry. Both the noisy and quiet supplies may be connected to the same voltage level. However, the noisy power supply would be connected to a separate pin from a quiet power supply. On the integrated circuit, the noisy power supply is connected to circuitry which generates or is subject to noise, while the quiet power supply is connected to relatively quiet circuitry. By separating the power supplies in this fashion, the circuitry connected to the quiet power supply is isolated somewhat from switching and other types of noise present or the noisy power supply.
In prior art devices, Vccq has been set at the same value as Vccn. It would be desirable to provide a circuit where Vccn is higher (e.g., 3.3 Volts) than Vccq (e.g., 2.5 Volts). The problem with such a scheme is that the gate drive (Vgs) to the output driver NMOS pull-down transistor of the output buffer may be at Vccq, while other parts of the circuit are at the higher Vccn level. Thus, it is difficult for the pull-down transistor to completely pull the output node to a digital low value. This degrades performance, especially when multiple outputs are switching. In addition, since the quiet mode supply voltage (Vccq) and the noisy mode supply voltage (Vccn) may not track each other, a worse case output delay can happen when Vccn=3.6 Volts and Vccq=2.25 Volts, as the output buffer tries to pull the output from 3.6 V to 0V. This problem did not exist in prior art devices where Vccn was always less than Vccq.
Another design challenge for output buffers in mixed-voltage systems is to support high and low signals in the Personal Computer Interface (PCI) bus protocol. Observing the PCI bus protocol requires large output driver transistors. This causes both Vcc sag and ground bounce problems that are not experienced in non-PCI systems. In addition, the larger output driver transistors result in larger die sizes.
In view of the foregoing, it would be highly desirable to provide an improved output buffer for use in mixed-voltage systems. More particularly, it would be highly desirable to provide an improved output buffer that supports a noisy internal supply voltage that is higher than a quiet internal supply voltage. Further, it would be highly desirable to provide both PCI compatibility and tolerance to high voltages (e.g., 5 Volts) in a mixed-voltage system.